Differential converters receiving a differential input signal X and providing a differential output signal Y are widely used in many devices. For example, voltage-to-current converters (VICs) are part of continuous-time analog-to-digital converters (ADC), filters, and other devices. Compared to voltages, currents are less sensitive to digital noise originating from a microchip substrate or from supply lines. In an environment where digital circuits and analog circuits are placed together on a single chip, processing signal currents by e.g., filtering, sampling, scaling, or the like, is more desirable than processing signal voltages. VICs are therefore very useful for modern circuit design.
Usually, VICs comprise differential transistor pairs and compensation circuits. For the application of VICs and for prior art designs, the following references are useful:
2! Babanezhad, J. N., Temes G.C.: "A 20 V four quadrant CMOS analog multiplier", IEEE Journal of Solid State Circuits, volume SC-20, pages 1158-1168, December 1985; PA1 3! Ismail, M., Fiez, T.: "Analog VLSI Signal and Information Processing", chapter 3.3, McGraw-Hill, 1994, ISBN 0-07-032386-0; PA1 4! Silva-Martinez, J.: "High-Performance CMOS Continuous-Time Filters", chapter 2, Kluwer Academic Publishers, 1993, ISBN 0-7923-9339-2; and PA1 5! Franca, J. E.: "Design of Analog-Digital VLSI Circuits for Telecommunications and Signal Processing", chapter 3.5 (pages 81-96), Second Edition, Prentice Hall, Englewood Cliffs, 1994, ISBN 0-13-203639-8.
The properties of the VIC influence the overall performance of the devices. The transfer function of the VIC, and in general that of any circuit, is limited by intrinsic nonlinearities of its elements. Especially, transistors have nonlinear transfer functions. The X-Y-transfer function between the input signal X and the output signal Y of a converter is generally not linear. The X-Y-transfer function can be expressed by a polynomial: EQU Y=k.sub.1 *X+k.sub.2 *X.sup.2 +k.sub.3 *X.sup.3 +. . .
where the symbol * indicates multiplication. A simple sinusoidal input signal X having the frequency f.sub.1 is transformed into an output signal Y having the fundamental frequency f.sub.1 and harmonics, such as f.sub.3 =3*f.sub.1 or other harmonics. The higher frequency parts (f.sub.2, f.sub.3, . . . ) of Y contribute to the Total Harmonic Distortion (THD). An input signal X having multiple frequencies is transferred into an output signal Y containing also sum and difference frequencies. That can lead to intermodulation distortions.
Relations between the magnitude of input signals X and supply voltages V.sub.s of circuits are well known. For example, input signal X oscillating with amplitudes .vertline.X.vertline.&lt;&lt;V.sub.s causes less distortion than when signal is in the range of V.sub.s, i.e. .vertline.X .vertline..apprxeq.V.sub.s. Amplifying elements, such as vacuum tubes, bipolar transistors, field effect transistors and other devices can operate sometimes in such V.sub.s regions where nonlinearities can be neglected. However, the trend in modern electronics is to use smaller and smaller supply voltages V.sub.s. Therefore, nonlinearities will occur and need to be compensated.
It is known in the art to compensate nonlinearities by, for example: (a) chaining differential transistor pairs in the VIC; (b) changing the bias for transistors depending on the input signal X; or (c) subtracting a compensation signal from output signal Y. In an example of (a), FIG. 2.2a in chapter 2.2. of 4! illustrates a converter with four transistors. In an example of (b), FIG. 25 in chapter 3.5.1 of 5! illustrates an amplifier employing a so-called adaptive biasing technique. In the approach (c) which has been disclosed with details in FIG. 3 of 1!, transistors 110 and 120 each receive a component of X (e.g., V.sub.1 and V.sub.2) and provide currents (e.g., I.sub.1 and I.sub.2). Currents I.sub.1 and I.sub.2 are still distorted and are linearized by subtracting current I.sub.3 and I.sub.4, resulting in output signal Y. However, currents I.sub.3 and I.sub.4 also contain the input signal X.
These prior art circuits can suffer from disadvantages well known in the art, such as for example, low gain, low signal-to-noise ratio (SNR), high power consumption and other problems. Hence, the present invention seeks to provide voltage-to-current converters (VICs) which mitigate or avoid some or all of these and other disadvantages and limitations of the prior art.